1. Field of the Invention
This invention relates to a power transmission system for transmitting a driving force of a prime mover to driving wheels via a stepped transmission.
2. Description of the Prior Art
Conventionally, a power transmission system of the above-mentioned kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 2000-65199. The power transmission system includes a five-forward-speed transmission having first to fifth speed input gears arranged on an input shaft thereof which is connected to an engine, in the mentioned order from the engine side, and first to fifth speed output gears arranged on an output shaft thereof, in constant mesh with the respective first to fifth speed input gears. These first to fifth speed input and output gears form first to fifth speed gear pairs, respectively. Further, the first and second speed output gears are rotatably supported on the output shaft, and a synchromesh is arranged on the output shaft between the two gears. The synchromesh is selectively put into meshing engagement with and disengagement from one of the first and second speed output gears, to connect and disconnect the one of the output gears to and from the output shaft, whereby the speed position of the transmission is set to the first speed position or the second speed position. Similarly, the third and fourth speed input gears are rotatably supported on the input shaft, and another synchromesh is arranged on the input shaft between the two gears. The synchromesh selectively connects and disconnects one of the third and fourth speed input gears to the input shaft, whereby the speed position of the transmission is set to the third speed position or the fourth speed position.
On the other hand, the fifth speed input gear is rotatably supported on the input gear shaft, and connected/disconnected to and from the input shaft by a shift clutch. The shift clutch is of a hydraulic type the engaging force of which is controllable. The shift clutch is arranged on an end portion of the input shaft on a side remote from the engine, and further, a shift cylinder for controlling the shift clutch is arranged at a location outward of the shift clutch. When the speed position of the transmission is set to the fifth speed position, the hydraulic pressure of the shift cylinder is maximized, whereby the shift clutch is completely engaged. On the other hand, in conducting up-shifting operations within a shift range up to the fourth speed by using the synchromeshes, the hydraulic pressure of the shift cylinder is reduced to a smaller value to allow the shift clutch to slide, whereby engine torque is transmitted and supplied to the output shaft as supplemental or compensating torque. In general, in torque transmission by a synchromesh alone, the transmitted torque is reduced to zero or a value close to zero during a time period from completion of synchronization of the synchromesh to establishment of complete meshing engagement thereof. The above control operation for the shift clutch is conducted to prevent this phenomenon of xe2x80x9ctorque transmission interruptionxe2x80x9d from causing a feeling of inertia travel which is peculiar to the synchromesh.
However, in the conventional power transmission system, it is required to arrange the hydraulic shift clutch and the shift cylinder for actuating the same on the input shaft along the same in addition to the synchromeshes so as to prevent the driver from having the inertia travel feeling during an up-shifting operation, which increases the length of the transmission in a direction along the input shaft and hence makes it impossible to make the transmission and the power transmission system including the same compact in size. Further, since the supplemental torque is supplied to the output shaft via the fifth speed gear pair, the supplemental torque becomes short particularly in shifting between low speed positions, causing a large step in torque changes, which degrades the speed position-shifting performance of the transmission.
It is an object of the invention to provide a power transmission system which is capable of positively preventing a feeling of inertia travel from being caused by torque transmission interruption which conventionally occurred when a dog clutch is engaged in shifting the speed position of a transmission, without increasing the axial length of the transmission, thereby improving the speed position-shifting performance of the transmission.
To attain the above object, the present invention provides a power transmission system for transmitting a driving force of a prime mover to driving wheels via a stepped transmission,
the stepped transmission including:
an input shaft connected to the prime mover,
an output shaft connected to the driving wheels,
a plurality of gear pairs for at least three speed positions of the stepped transmission, each formed by an input gear arranged on the input shaft, and an output gear arranged on the output shaft and in mesh with the input gear, with one gear of the input gear and the output gear being rotatably supported on the input shaft or the output shaft,
connecting/disconnecting clutch means of a dog clutch type, for selectively connecting and disconnecting the one gear of the input gear and the output gear of each of the plurality of gear pairs to and from the input shaft or the output shaft rotatably supporting the one gear, to thereby establish a speed position of the stepped transmission by a connected one of the gear pairs including the connected one gear, and
a driving force assist mechanism for supplying the driving force of the prime mover to the output shaft when a shift in speed position is carried out by the connecting/disconnecting clutch means,
wherein the driving force assist mechanism comprises:
an auxiliary shaft arranged in parallel with the input shaft and the output shaft;
an intermediate gear integrally formed with the auxiliary shaft and connected to the input shaft;
an auxiliary gear rotatably supported on the auxiliary shaft and connected to the output shaft; and
an assist clutch for fastening the auxiliary gear to the auxiliary shaft to thereby transmit the driving force of the prime mover from the input shaft through the auxiliary shaft and the auxiliary gear to the output shaft.
According to this power transmission system, a plurality of gear pairs for at least three speed positions are each formed by an input gear arranged on an input shaft connected to the prime mover, and an output gear arranged on an output shaft connected to driving wheels and in mesh with the input gear, and these gear pairs are selectively connected/disconnected by connecting/disconnecting clutch means of a dog clutch type, to establish a speed position of the transmission by the connected one of the gear pairs. Further, the driving force of the prime mover is transmitted to an auxiliary shaft of a driving force assist mechanism which is arranged in parallel with the input shaft and the output shaft, via the input shaft and an intermediate gear integrally formed with the auxiliary shaft. Then, when the speed position is shifted by using the connecting/disconnecting clutch means, an assist clutch of the driving force assist mechanism fastens an auxiliary gear rotatably supported on the auxiliary shaft to the auxiliary shaft, whereby the driving force transmitted to the auxiliary shaft from the input shaft is transmitted to the output shaft via the engaged auxiliary gear.
As described above, according to the power transmission system, when the speed position of the transmission is shifted by using the connecting/disconnecting clutch means, the driving force of the prime mover is transmitted to the output shaft via the auxiliary gear fastened to the auxiliary shaft by the assist clutch to supply a supplemental or compensating driving force. Therefore, under the condition of the supplemental or compensating driving force being supplied, the connecting/disconnecting clutch means is connected i.e. put into meshing engagement, so that even if transmission of the driving force (torque) via the connecting/disconnecting clutch means is interrupted during the connecting operation thereof, it is possible to prevent the driver from having the feeling of inertial travel. Further, since the auxiliary shaft is arranged in parallel with the input shaft and the output shaft, and at the same time, the intermediate gear, the auxiliary gear, and the assist clutch are arranged on the auxiliary shaft, the above advantageous effects can be obtained without increasing the axial length of the transmission.
Preferably, the auxiliary gear comprises a plurality of auxiliary gears having respective different numbers of gear teeth, and selectively fastened to the auxiliary shaft by the assist clutch.
According to this preferred embodiment, the auxiliary gear comprises a plurality of auxiliary gears having respective numbers of gear teeth, which enables the magnitude of supplemental torque to be set to one of a plurality of steps according to a selected one of the auxiliary gears. Therefore, by selecting one of the auxiliary gears according to a speed position to which the shift should be made, the supplemental torque can be supplied just in an appropriate amount. As a result, it is possible to reduce a torque step occurring during the shift in speed position, thereby improving the speed position-shifting performance of the transmission.
More preferably, the plurality of gear pairs include a gear pair for an N-th speed position and a gear pair for an (N+1)-th speed position one speed position higher than the N-th speed position, and when the stepped transmission is shifted in speed position from the N-th speed position to the (N+1)-th speed position, one of the plurality of auxiliary gears which can transmit the driving force from the input shaft to the output shaft at a change gear ratio approximately equal to a change gear ratio at which the gear pair for the (N+1)-th speed position transmits the driving force is fastened to the auxiliary shaft by the assist clutch, and then after the connecting/disconnecting clutch means effects the disconnection of the gear pair for the N-th speed position, the connecting/disconnecting clutch means effects the connection of the gear pair for the (N+1)-th speed position.
According to this preferred embodiment, when speed position of the transmission is shifted from the N-th speed position to the (N+1)-th speed position, the auxiliary gear is fastened to the auxiliary shaft, and then, after disconnecting the gear pair for the N-th speed position, the gear pair for the (N+1)-th speed is connected. Therefore, the gear pair for the (N+1)-th speed position can be connected under the condition of the supplemental torque being positively supplied to the output shaft via the auxiliary gear. Therefore, even during an up-shift which particularly tends to cause the driver to have the feeling of inertia travel, it is possible to positively prevent the feeling from being given to the driver. Further, as the auxiliary gear fastened in the above process, there is selected one capable of transmitting the torque (driving force) from the input shaft to the output shaft at a change gear ratio approximately equal to a change gear ratio at which the gear pair for the (N+1)-th speed position transmits the torque, it is possible to control the change gear ratio of the transmission during supplemental supply of the torque to approximately the same as that of the gear for the (N+1)-th speed position. Therefore, the connecting engaging) operation of the connecting/disconnecting clutch means can be carried out under the condition of a small difference in rotational speed between the input or output gear of the gear pair for the (N+1)-th speed position and the input shaft or output shaft which are to be connected to each other. This reduces the torque load on the connecting/disconnecting clutch means, and hence it is possible to reduce the torque capacity of the clutch means, and at the same time, reduce the time for a shift in speed position, thereby further improving the speed position-shifting performance of the transmission.
Preferably, the intermediate gear is in mesh with one of the plurality of gear pairs, and the auxiliary gear is in mesh with another of the plurality of gear pairs.
More preferably, the intermediate gear is in mesh with one of the plurality of gear pairs, and the plurality of auxiliary gears are in mesh with respective other ones of the plurality of gear pairs.
According to these preferred embodiments, as the gears for transmitting torque from the input shaft via the intermediate gear to the auxiliary shaft, existing gears of the conventional transmission are commonly used, which makes it possible to reduce the number of gears, and at the same time reduce the axial length of the transmission.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.